Many residential and commercial fires result from causes that are generally, and sometimes inaccurately, described as faulty electrical wiring, or electrical circuit failures. One common cause of such failures is overheating of or by an extension cord because of circuit overload or other conditions, separately or in combination. For example, when a temperature is reached within or outside the extension cord at which the synthetic or rubber compound insulation melts or decomposes, a heated interior wire may become exposed and come in contact with and ignite flammable material. Also, insulation failures can also allow high amperage arcing to occur between interior wires. The result is a type of fire which is known to cause loss of a number of lives and much residential and commercial structure damage each year. Electrical circuits and interconnecting cords and cables are designed to function with an expected degree of resistive heating, and to accommodate temperature buildup as heat is conducted along the circuits from one point to another. A long extension cord, for example, transfers heat along its conductive wiring from the outlet or source from which it derives current, and from an appliance or device at which electrical power is consumed. Combinations of operating factors thus can result in overheating of an extension cord, for example, and temperatures can reach a level at which decomposition begins and incipient danger exists.
Electrical codes require protection against electrical overload conditions in permanent installations by compelling inclusion of fuses or circuit breakers in the wiring system between the main power supply and the points of usage in a residential or commercial building. There are, nonetheless, a number of types of potential wiring-originated fires that are not precluded by fuse or circuit breaker protection, including those mentioned above.
Although product manufacturers and electrical equipment code requirements provide instructions as to preferred and limiting conditions for use of components such as receptacles, extension cords, and circuit breakers, there can be no guarantee that users will comply with these stated instructions. Extension cords, for example, are nominally intended to carry from 1200-1600 watts, but users may in practice drastically overload an extension cord in a number of ways. For example, there is a tendency to use extension cords and circuits beyond their rated capacities, as by attaching a cord to an appliance of higher power than is nominally permitted. At the receptacles and outlets, there is a tendency to attach multiple devices, and if too many are coupled in this, may exceed the load carrying capacity of the receptacle even though individual appliances and devices may not demand much power individually.
Special problems can arise from environmental and building conditions. For example, when exterior temperatures in a region approach or exceed 100.degree. F., the temperature in attics, under-roof crawl spaces, and inside walls can approach considerably higher levels. In these uninsulated interior spaces the temperature can be 40-60.degree. F. higher than the outside air temperature from the sun heating the exterior surfaces, which transmit radiant energy to heat the interior spaces. Extensions and other wiring are often placed in such spaces. The insulation of such wiring can approach threshold levels merely because of extremely high ambient temperature, because of being in a confined space, or because of an increase in resistance heating caused by increased ambient temperature. The resistance of copper conductors increases by a factor of 0.0047 for each degree Celsius above zero, thus increasing resistive heating with temperature when current is constant. The effect can be compounded if multiple wires are run in the same space or, as in the case of extension cords, the wire is bundled or coiled to fit within a limited space. Under these circumstances, relatively minor overheating of lines, receptacles and/or appliances can substantially increase danger of combustion.
Electrical fuses protect substantially only against current overloading, as do circuit breakers. Furthermore, circuit breakers can malfunction and fail to operate. Under such conditions, they may overheat, and even if they do not themselves fail, they act as a heat source for interconnected elements in the wiring system. The larger the gauge of the copper wires used in an extension cord, for example, the more current it can carry for each degree of temperature rise. At the same time, the copper, which is an extremely good thermal conductor, becomes more efficient in transferring heat energy along its length. Consequently, there can be bi-directional interaction between different points in an electrical system, so that a fire need not necessarily arise at some point of malfunction, but may be initiated at a remote location that is in effect a weak link in the system. It is evident that protection is needed whether there is an overheating danger from causes other than electrical malfunction, or some misoperation or misuse of electrical equipment. It is obvious, furthermore, given the large installed inventory of receptacles, outlets, circuit breakers and fuses, as well as the widespread employment of semipermanent adapter outlets and extension cords, that the patterns of usage in residential and commercial buildings will not substantially change. Consequently, the dangers inherent in many usage habits will remain unless a convenient means is found for protecting against fire danger from these causes. For these and other reasons, there is a need for compact and inexpensive safety elements which cooperate with standard electrical circuitry to protect against the types of failures in electrical wiring systems that endanger individuals and cause damage or destruction to buildings.